Known two dimensional code readers are of two types:
The first type--Flying spot scanners.sup.7, employing a focused laser spot, scanning two dimensional codes both horizontally and vertically.
The second type--CCD based code readers.sup.9, where the scanning is obtained electronically, using an imaging lens and an area image sensor, or a linear CCD array, utilizing either a scanning mirror or a manual scanning method for scanning in the vertical direction.
Spatial resolution is one of the most significant parameters of any bar code reader. It determines the narrowest bar or space, that can be decoded. Resolution is typically measured in units of mil (0.001 inch). As a rule, CCD-based code readers have higher resolution than laser scanners, which can be explained as following: The spatial resolution of flying spot laser scanners is determined by the minimal effective spot size of light, of the laser diode. This spot size is determined by the cross section of the focused laser beam in the working range, which is relatively large because of the diffraction limited beam waist in the center of the working range and moreover so at the ends of the working range. In CCD based readers, the optical projection system creates the code's image on the surface of the CCD detector array.sup.1. Optimal optical projection systems have Contrast Transfer Function (CTF), which allow to utilize effectively CCD sensors with a given pixel size so that the reader's spatial resolution at the surface of the code is determined solely by the size and number of the CCD array pixels.
Optical projection systems for reading two dimensional codes like PDF417 need to have a wide field of view angle. Objectives of this kind have typically complicated optical schemes using 7-8 single optical elements, leading to a high component cost. Moreover, the spatial resolution of such objectives is further limited at large field of view angles by off axis aberrations. Thus, the increase of spatial resolution of code readers, requires a new projection system design, that can yield high spatial resolution characteristics for a large area in the object's space, within the demanded working range.
Working range (WR) is another significant parameter for bar code readers. It specifies the longitudinal distance range within which reading and decoding can be accomplished. Conventional CCD bar code scanners have a limited WR, determined by the limited depth of field of the optical system. Thus devices of that type are usable for reading bar codes at fixed distances only. This limitation of CCD bar code readers can be overcome by means of autofocus optics.sup.4,6,14.
Low reading sensitivity to the reader's holding position, is another significant parameter for code readers. The readers of two dimensional codes, available presently, are quite limited with this regard. Any small deviation from the optimal scanner position, prohibits code reading. The new type of reader, of the present invention, is capable of reading codes essentially at any rotational angle position. Furthermore, high speed processing techniques accelerate the code reading and decoding process.